This invention relates to antennas comprising an array of helical radiators and, more particularly, to a helical array antenna, or a feed in the case of a reflector antenna, wherein distances between the radiators and a mounting base, such as a ground plane or feed of the antenna, are staggered in an amount equal approximately to one turn of a helix.
A helical antenna is formed of an elongated electrical conductor, such as a wire or rod, which is wound in spiral fashion upon a central electrically-insulating support to form a helix wherein the support lies along an axis of the helix. Generally, the helix extends outward from a mounting base, such as a ground element or ground plane disposed behind the helix and perpendicularly to an axis of the helix. Upon application of an RF (radio frequency) signal, between a back end of the conductor and the ground element, the helix acts as a slow-wave structure and radiates an electromagnetic wave from the helix in the manner of an end-fire array. There results a relatively narrow beam of radiant energy which is directed along the helix axis in the forward direction.
To increase the power and directivity of the beam, a plurality of helical radiators may be arranged side-by-side along a common ground plane to produce a resultant beam which is a composite of the beams of the individual radiators. Alternatively, a beam can be given a desired shape by placing a reflector in front of a helical radiator. When several beams are to be provided, an antenna feed is constructed of several helical radiators which face a common reflector, and each radiator may be operated at slightly different radiation frequencies which distinguish the signals of the respective beams. In both of the foregoing examples, there is provided an array of helical radiators arranged side-by-side.
In such an array of radiators, each radiator retains its radiation characteristic if it is positioned at a sufficient distance from a neighboring radiator to insure no more than an insignificant amount of mutual coupling between the radiators. A minimal spacing, d, is given approximately by the product of the wavelength of the radiation multiplied by the square root of (G/4.pi.) where G is the gain of an individual helical radiator.
A problem arises in a situation wherein it is desired to space two helical radiators more closely together than the minimum spacing, d. There results a mutual coupling which degrades the end-fire radiation pattern of each helical radiator. This is disadvantageous in a situation wherein it is desired to mount the radiators as close as possible to the focal point of a reflector so as to generate, for example, equally formed beams of radiation at each of separate frequency bands to be transmitted (or received) by the antenna. Also, the feed for a reflector antenna may have closely positioned radiators to generate the single beam of radiation having far more power than is available from a single radiator. In either of the foregoing examples, the minimum spacing between radiators has been limited, as noted above, to avoid excessive mutual coupling between the radiators. As a result, there is less control over the beam pattern than would be desirable.